1. Field of the Invention
The present invention relates to an electric heating cable or an electric heating tape having insulating sheaths of polytetrafluoroethylene that are arranged in a layered structure.
2. Description of the Background Art
Heating cables in a coaxial arrangement, wherein the heat conductor is enclosed by a fluoropolymer as insulating material, are known (DE-A 28 50 722) for a very wide range of applications, for example including the heating of aggressive media. This insulation is covered by a copper wire braid, where the individual copper wires are additionally nickel-plated for corrosion resistance. This braid of copper wires is the electrical ground conductor for the cable, which is provided in the cable to preclude the risk of accidents, for example resulting from such causes as short circuits in the electrically conductive part. The ground conductor is covered by an outside plastic jacket that is made, for example, of a fluoropolymer to protect against aggressive media in the environment. The advantage of a coaxial arrangement structured in this way, in addition to the wide range of applications for this cable that result from the use of materials resistant to high temperatures and aggressive media, is that such cables can be manufactured in almost any desired length with great flexibility.
The same is true for known electric heating tapes (GB 2 092 420 A, GB 2 130 459 A), which are used for example for pipe heaters, and also on steam-cleaned pipes to maintain or raise the temperature. Lastly, so-called self-regulating heating tapes with a semiconductor heating element are also in use. Since the emission of heat is automatically controlled here as a function of the ambient temperature, such heating tapes are especially suitable for use in areas where explosion hazards exist.
However, when heating cables are used as, for example, coaxial types, it frequently happens that the outer jacket is so severely crushed by external forces so that the insulation is forced away from the heating conductor, that either the ground conductor and heating conductor contact one another or that the insulating distance between the heating and ground conductors has become so small that corona or spark discharges occur. Moreover, the damage can cause broken wires of the ground conductor to penetrate the insulation and thus lead to failure of the entire heating cable. These criteria must be paid particular attention in heating cables that are used in explosion-proof systems and that are thus subject to special safety requirements as preventive explosion protection. However, these criteria must also be taken into account with regard to applicable standards (DIN VDE 0170/0171, EN 50014 and EN 50019), which for example require a ground conductor that ensures adequate coverage of the surface of the conductor insulation as well as separate crush testing followed by testing of the insulating properties of the conductor insulation. Increasing the wall thickness of both the insulation and the outer jacket to avoid these problems provides no additional benefit here; moreover, these measures significantly increase the diameter of the cable as a whole and also increase costs due to the larger quantity of fluoroplastic used.
An electric heating cable with a coaxial layered structure that is resistant to external mechanical stresses is known from, for example, DE-ES 101 07 429. A glass ceramic tape layer in the layer structure above the conductor insulation of this cable is intended to offer protection from external mechanical damage in conjunction with a similarly air-permeable reinforcing layer. Air-impermeable layers of an extrudable fluoropolymer are provided on both sides of these two layers so that an air cushion can form between them. Aside from this costly layered construction, which also increases the cable diameter, the intentionally created air cushion inside the cable leads to significant impairment in the conduction of heat away from the heating conductor to the cable surface, and thus to degradation of the efficiency of the heating cable itself.
In order to avoid this but still satisfy the requirements of the applicable standards for adequate resistance to external impact and compressive stresses, it has already been proposed (EP 0 609 771 B1) to provide one or more layers of a tape made of plastics having high mechanical strength, such as polyimide, above and/or below the ground conductor in an electrical heating cable of the generic type. Such a wrapping is capable of withstanding high compressive stresses, external impacts are absorbed in a dammed manner, and damage to the conductor insulation is avoided.